Biomarkers and Stable Isotopes in Neogene Coals

Research project P 14245 Biomarkers and Stable Isotopes in Neogene Coals Reinhard GRATZER 08.05.2000 Studies in hydrocarbon composition of terrigenous organic matter revealed the importance of individual biomarkers in reconstructing the ancient biosphere. Measurement of the stable isotopic composition of organic matter and carbonates is widely used for the assessment of paleoclimates and in paleoenvironmental studies. In order to overcome difficulties in the interpretation of the results, the combined use of organic geochemical and stable isotope data as proxies for paleocology and climatic changes during Earth`s history attained increasing significance. The bulk of organic matter in coals is derived from faily uniform biological source materials (land plants), the remains of which can be identified by microscopic observations (maceral analyses). Free hydrocarbons derived from plant resin polymers are present in considerable quantities in coals of low maturity, and several of these hydrocarbons can be considered as "chemical fossils". During the late Tertiary, decreasing temperatures and atmospheric carbon dioxide levels, accompanied by ecological chances (increasing predominance of Angiosperms), have been proposed. For all these reasons, lignites covering the time interval between the Egerian and the Pliocene, are selected for the proposed interdisciplinary study on organic geochemistry and stable carbon isotope rations of organic matter. Hydrocarbon and stable isotopic compositions of total organic carbon and specific biopolymers (resins, cellulose) will be investigated on narrowly sampled coal seams of the Oberdorf deposit. The relationship between maceral composition, organic geochemistry, bulk geochemical data and stable carbon isotopic ratios are expected to provide conclusive information about the major factors affecting the isotopic composition of terrigenous organic matter and carbonate. Based on the implementations from this part of the study, the reconstruction of environmental changes during the Neogene will be achieved by the extension of the planned stable isotope and organic geochemical analyses to selected Tertiary lignite deposits of different age from the Eastern Alpine region. It is anticipated that the combination of these results with climatic proxies from previous studies, and from ongoing research on stable isotopic composition of calcareous fossils from the study area, will address fundamental questions regarding the biogeochemical carbon cycle.

The combined use of organic geochemical and stable isotope data as proxies for paleoecology and climatic changes during Earths history achieves increasing attention. Tertiary lignites within the Eastern Alps and along their margins, covering the time interval from the Late Oligocene to Pliocene, are the focus of an interdisciplinary study on hydrocarbon composition and stable carbon isotope ratios. Relationships between maceral composition, organic geochemistry, palynology, and carbon isotopic ratios of total organic matter and fossil wood, investigated so far on densely sampled coal seams, provide information on the major factors affecting the isotopic composition of coals. Good correlations between palynomorphic spectra, the ratio of biomarker concentrations from conifers versus angiosperms, and tissue preservation index, suggest that organic geochemical data can be used for the reconstruction of vegetational changes. Significant differences in the peat-forming vegetations have been revealed by the molecular composition of hydrocarbons present in the investigated coal seams. The differences reflect vegetational changes within the mire caused by variations in (ground)water table and pH-conditions. The respective coal facies could be confirmed by the hydrocarbon composition of the different coal seams through relationships with differences in gelification of plant tissue, sulfur content and pH within the mire. The results further suggest that gelification of plant tissue may be governed by the activity of anaerobic rather than by aerobic bacteria. A general influence of the floral assemblage on the carbon isotopic composition of the coals is proposed. Stable carbon isotopic ratios observed within the lignite seams are further affected by the amount of isotopically light lipids (plant waxes, resins, etc.) and the activity of bacteria within the mire. The carbon isotope data of fossil wood remains are consistent with their chemotaxonomical classification as angiosperms and gymnosperms, respectively. Higher isotopic differences between cellulose and fossil wood, compared to that found in modern trees, are in agreement with the smaller effect of 13C discrimination for cellulose when compared with wood during decomposition. Compared with the coals, the carbon isotopic compositions of wood and extracted cellulose are affected to minor extent by microbial activity. The long-term decline of the average carbon isotope ratios from cellulose of gymnosperms from Early Miocene to Pliocene lignite seams is consistent with data from previous studies on fossil wood from coals of the Lower Rhine Basin. The results are interpreted to reflect changes in the carbon isotopic composition of land plants during the Neogene due to a general decrease in mean annual temperature and precipitation, as implied from paleobotanical records in coals and sediments from central Germany.